If the mains power supplying an elevator fails, the elevator will stop running. If the elevator is between floors, the passengers are unable to get out of the elevator and this can cause impatience and anxiety. Most elevators are provided with an alarm button inside or outside the elevator cab which can be pressed by passengers trapped in the elevator or people outside the elevator. This may cause an alarm to sound outside the elevator to alert help and/or may be connected, by telecommunication, to a help center so that the trapped passengers can communicate with someone outside of the elevator and call for help. The alarm button will have its own power supply e.g. a battery so that it remains in action even when the mains power fails.
To avoid passengers being trapped in an elevator for any length of time, many modern elevators are now provided with a back up power supply in the form of a battery or accumulator which is switched on either automatically or by pressing a button within the elevator in the event of a mains power failure. The power from the battery is sufficient for the elevator controller to be able to bring the elevator to the nearest floor. When the elevator arrives at the floor the doors can be opened and the passengers can exit the elevator.
Most elevators comprise an elevator car suspended in a hoistway or shaft on steel ropes or cables which run over a pulley at the top of the shaft and which are attached at the other end to a counterweight. A main motor is provided to drive the elevator car in accordance with instructions from an elevator controller.
In the event of a mains power failure, the motor ceases running and the brake is applied to prevent the elevator falling to the bottom of the hoistway or being shot up the hoistway as the counterweight falls. This will result, in many cases, in the elevator being suddenly brought to a stop between floors.
In some elevators, a rescue mechanism allows the brake to be released by providing power from an emergency power supply such as a battery to the controller. Depending on the relative weights of the elevator car and the counterweight the car will either move up or down until it reaches the next floor. A sensor will detect when the elevator reaches the floor and will re-apply the brake and the doors can be opened to let the passengers out.
US Patent Publication No. 20040020726 discloses such an emergency operation. When the car is trapped between floors, the passengers press an emergency button which releases the brake allowing the car to move to the next floor.
U.S. Pat. No. 6,264,005 also teaches such a system in which the actual speed of the car in moving to the next floor is not (as in US 2004002726) merely dependent on the difference in weight between the car (which is dependent on the passenger load) and the counterweight. In this patent, the rescue operation during power failure is controlled by controlling a speed and torque of a permanent magnet type synchronous motor with its electricity generating power.
Both systems do, however, rely on an imbalance between the car and the counterweight to bring the elevator to the next floor in the case of a car stopping between floors due to power failure. This means that the rescue operation will not work where there is no load imbalance. Furthermore, the elevator will only be able to move in one direction (depending on the relative weights of the car and the counterweight) and will not necessarily move to the nearest floor. If the rescue operation is also to work where there is no load imbalance drive support is required—i.e. a drive powered by the emergency power supply must be able to drive the elevator to the next floor.
In all systems with a rescue operation, circuitry is provided which allows the parts of the system needed to implement the rescue operation to be supplied with power from an emergency power supply in the event of mains power failure. The emergency power supply is usually a battery or accumulator. The circuitry thus usually includes a switched mode power supply for switching from mains power to battery power.
Most such systems use an uninterruptible power supply powered by batteries or an emergency generator. In the event of a mains power failure, these devices will generate the same voltage level as was provided by the mains supply.
Such arrangements are large and expensive, and require fairly complex circuitry and relatively large components requiring more board space and more connections. There is, therefore, a need for a simple, effective switched mode power supply circuit which allows power supply to a drive circuit for an elevator to switch easily from mains power to battery power in, e.g. the event of a power failure so that the elevator can be driven to the next floor.